This invention relates generally to an improved method of producing writing, drawing, marking, recording, and like inscribing instruments and more particularly to the inscribing points or nibs thereof which are of bar shape and are made of thermoplastic resins. More specifically, the invention concerns an improved method of producing nibs of this character having hollow capillary interiors.
In general, an inscribing nib in which capillarity is utilized must fulfill the following two important requirements.
The first requirement is that the nib must supply a steady outflow of ink to produce a clear and distinct inscription without discontinuities and without causing the nib tip to become scratchy.
The second requirement is that the nib must possess sufficient "ink-retaining" ability (capillarity) such that the ink drawn into the nib interior will not drip under its own weight from the outer tip of the nib.
The first of these requirements can be satisfied by so fabricating the nib that its total cavity space constituting the ink passage in the cross section of the nib (total sum of the cross-sectional areas of the plurality of spaces) will be large thereby to permit the ink to flow at an amply high rate.
The second requirement can be satisfied by making the parts of the ink passage in cross section as narrow as possible, that is, making the transverse width, or wall-to-opposite-wall dimension of the parts of the ink passage as seen in cross section as small as possible, thereby to produce high capillarity and good ink-retaining ability.
In order to obtain a nib fulfilling the above described two requirements and, moreover, having uniform inscribing performance (i.e., producing clear inscription without dripping) when any part around the circumference of the nib tip contacts the surface being inscribed, the nib is preferably so fabricated that, within limits imposed by the necessity of providing a capillarity sufficient for preventing dripping of the ink, the ink-conducting passage will afford maximum total space as viewed in cross section of the nib and will be of a pattern, or distribution, which is of uniform density over the entire cross section.
However, in the production of a nib of the instant character, certain characteristics fundamentally required of the nib, other than those described above, must also be considered. The more important of these characteristics are strength to withstand the stress imparted to the nib during inscribing, wear-resisting performance for withstanding a long period of use in inscribing, and the property of imparting a feeling of smooth gliding of the nib tip during inscribing.
Examples of nibs formed from thermoplastic resins and having the above mentioned strength, wear resistance, and smooth inscribing feel and disclosed in the specifications of U.S. Pat. Nos. 3,338,216; 3,520,629; and 3,614,247. In each of these nibs, however, the ink conducting passage is of a shape wherein a space of very small width as seen in cross section extends in the axial centerline direction of the nib. For this reason, the total area of the cross-sectional space constituting the ink-conducting passage is small, whereby an ink outflow rate which can amply keep up with the inscribing speed cannot be obtained. As a result, the nib tip tends to become scratchy, and the inscription readily becomes discontinuous. This defective operation occurs particularly in the case of nibs of diameters less than approximately 1 millimeter.
Heretofore, for the production of the marking nibs of the type under consideration, one practice has been to force a molten thermoplastic resin through an extrusion die having a plurality of orifices of the same diameter arranged in close proximity to each other. The resin emerges from the die in the form of filaments of substantially equal diameter, and while the surfaces of these filaments are still in a molten or semi-molten state, they are caused to adhere longitudinally to each other to provide an elongate bar having a plurality of ink-conducting passages or channels of capillary dimensions extending longitudinally therethrough.
According to this previous method, the cross section shapes and dimensions of the ink channels are determined by the filaments of substantially uniform diameter. Since, in practice, there are limitations upon the possible relative cross sectional arrangements of such uniform diameter filaments, the capillary action of each channel tends to become irregular in the cross sectional direction of the nib produced.
The nib of the above described character is objectionable from the standpoint of its ink-conducting ability, that is, the uniformity of ink flow through and out of the nib when in contact with a writing or marking surface. The ink retaining ability of the nib in question is also poor, so that when the instrument is brought back to a writing or marking position after having been left in an upside-down disposition, for instance, normal writing or marking cannot be effected unless the capillary channels of the nib become refilled with the ink upon elapse of a certain length of time. This is because the channels became devoid of the ink while the instrument was held in the upside-down disposition.
Thus, according to the previous method, the nibs which have channels each having constant capillarity throughout its cross-sectional area and which are futher satisfactory in the ink retaining ability can be produced only after a long trial-and-error process of finding out the optimum diameter and the optimum arrangement of extrusion orifices to be formed through a die. While the capillarity of the channels can theoretically be rendered cross sectionally constant by use of extremely fine filaments, this scheme is not quite practicable because then the extrusion die requires an inordinately large number of orifices of minute diameter.